Despite the Hype, it's a good book, but NOT the most important for playing ArM. The best additions are the faerie spells, about 50% of the Hermetic spells are flawed or out of paradigm.

The best 3rd-Ed supplement so far. Although the spells need some debugging, the ideas and rules are overwhelming. Give color to your labs and libraries

Good ideas for ruling at Tribunals.

Really hit-or-miss sourcebook. Every troupe should vote on each idea before allowing it.

As a compendium of House Rules, it's interesting reading. It's probably of limited usefulness to most people; the laboratory and book rules, as well as the sections on the Peripheral Code, are the most worthwhile.